Polyaxial pedicle screw having a threaded and tapered compression locking mechanism

ABSTRACT

A polyaxial orthopedic device for use with rod implant apparatus includes a screw having a curvate head, a two-piece interlocking coupling element which mounts about the curvate head, and a rod receiving cylindrical body member having a tapered socket into which both the screw and the interlocking coupling element are securely nested. The interlocking coupling element includes a socket portion which is slotted and tapered so that when it is radially compressed by being driven downwardly into the tapered socket in the cylindrical body it crush locks to the screw. The securing of the rod in the body member provides the necessary downward force onto the socket portion through a contact force on the top of the cap portion. Prior to the rod being inserted, therefore, the screw head remains polyaxially free with respect to the coupling element and the body. In a preferred embodiment, the cap portion and the socket portion are formed and coupled in such a way that when the cap portion is compressed toward the socket portion, there is an additional inward radial force applied by the cap portion to the socket portion, thereby enhancing the total locking force onto the head of the screw.

CROSS-REFERENCE TO PRIOR APPLICATION

This application is a continuation of U.S. patent application Ser. No.09/992,612, filed on Nov. 13, 2001, which is a continuation of U.S.patent application Ser. No. 09/774,915, filed on Jan. 30, 2001, now U.S.Pat. RE 37,665, which is a reissue of U.S. Pat. No. 5,882,350 issuedfrom U.S. patent application Ser. No. 09/002,535 filed on Jan. 2, 1998,which is a continuation-in-part of U.S. patent application Ser. No.08/843,972 filed on Apr. 17, 1997, now U.S. Pat. No. 5,888,204, which isa continuation-in-part of U.S. patent application Ser. No. 08/632,560filed on Apr. 15, 1996, now U.S. Pat. No. 5,725,588, which is acontinuation-in-part of U.S. patent application Ser. No. 08/421,087filed Apr. 13, 1995, now U.S. Pat. No. 5,520,690. U.S. patentapplication Ser. No. 09/002,535 filed on Jan. 2, 1998, now U.S. Pat. No.5,882,350 is also a continuation of U.S. patent application Ser. No.08/835,909 filed Apr. 10, 1997, now abandoned, which is acontinuation-in-part of prior application U.S. Ser. No. 08/663,383,entitled “A Polyaxial Pedicle Screw”, filed Jun. 13, 1996, now U.S. Pat.No. 5,669,911 which is a continuation-in-part of U.S. patent applicationSer. No. 08/559,196 filed Nov. 13, 1995, now abandoned, and which, inturn, was a continuation-in-part of Ser. No. 08/421,087, filed Apr. 13,1995, now issued U.S. Pat. No. 5,520,690, entitled “An Anterior SpinalPolyaxial Locking Screw Plate Assembly”. The disclosure of each of theseprior applications is hereby incorporated by reference in theirentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a polyaxial screw and couplingapparatus for use with orthopedic fixation systems. More particularly,the present invention relates to a screw for insertion into spinal bone,and a coupling element polyaxially mounted thereto, via a two-pieceinterlocking coupling element having a socket portion and a threadedcompression member, for coupling the screw to an orthopedic implantationstructure, such as a rod, therein enhancing the efficacy of the implantassembly by providing freedom of angulation among the rod, screw andcoupling element.

2. Description of the Prior Art

The bones and connective tissue of an adult human spinal column consistsof more than 20 discrete bones coupled sequentially to one another by atri-joint complex which consist of an anterior disc and the twoposterior facet joints, the anterior discs of adjacent bones beingcushioned by cartilage spacers referred to as intervertebral discs.These more than 20 bones are anatomically categorized as being membersof one of four classifications: cervical, thoracic, lumbar, or sacral.The cervical portion of the spine, which comprises the top of the spine,up to the base of the skull, includes the first 7 vertebrae. Theintermediate 12 bones are the thoracic vertebrae, and connect to thelower spine comprising the 5 lumbar vertebrae. The base of the spine isthe sacral bones (including the coccyx). The component bones of thecervical spine are generally smaller than those of the thoracic andlumbar spine. For the purposes of this disclosure, however, the wordspine shall refer only to the cervical region.

Referring now to FIGS. 1, 2, and 3, top, side, and posterior views of avertebral body, a pair of adjacent vertebral bodies, and a sequence ofvertebral bodies are shown, respectively. The spinal cord is housed inthe central canal 10, protected from the posterior side by a shell ofbone called the lamina 12. The lamina 12 includes a rearwardly anddownwardly extending portion called the spinous process 16, andlaterally extending structures which are referred to as the transverseprocesses 14. The anterior portion of the spine comprises a set ofgenerally cylindrically shaped bones which are stacked one on top of theother. These portions of the vertebrae are referred to as the vertebralbodies 20, and are each separated from the other by the intervertebraldiscs 22. The pedicles 24 comprise bone bridges which couple theanterior vertebral body 20 to the corresponding lamina 12.

The spinal column of bones is highly complex in that it includes overtwenty bones coupled to one another, housing and protecting criticalelements of the nervous system having innumerable peripheral nerves andcirculatory bodies in close proximity. In spite of these complexities,the spine is a highly flexible structure, capable of a high degree ofcurvature and twist in nearly every direction. Genetic or developmentalirregularities, trauma, chronic stress, tumors, and disease, however,can result in spinal pathologies which either limit this range ofmotion, or which threaten the critical elements of the nervous systemhoused within the spinal column. A variety of systems have beendisclosed in the art which achieve this immobilization by implantingartificial assemblies in or on the spinal column. These assemblies maybe classified as anterior, posterior, or lateral implants. As theclassifications suggest, lateral and anterior assemblies are coupled tothe anterior portion of the spine, which is the sequence of vertebralbodies. Posterior implants generally comprise pairs of rods, which arealigned along the axis which the bones are to be disposed, and which arethen attached to the spinal column by either hooks which couple to thelamina or attach to the transverse processes, or by screws which areinserted through the pedicles.

“Rod assemblies” generally comprise a plurality of such screws which areimplanted through the posterior lateral surfaces of the laminae, throughthe pedicles, and into their respective vertebral bodies. The screws areprovided with upper portions which comprise coupling elements, forreceiving and securing an elongate rod therethrough. The rod extendsalong the axis of the spine, coupling to the plurality of screws viatheir coupling elements. The rigidity of the rod may be utilized toalign the spine in conformance with a more desired shape.

It has been identified, however, that a considerable difficulty isassociated with inserting screws along a misaligned curvature andsimultaneously exactly positioning the coupling elements such that therod receiving portions thereof are aligned so that the rod can be passedtherethrough without distorting the screws. Attempts at achieving properalignment with fixed screws is understood to require increased operatingtime, which is known to enhance many complications associated withsurgery. Often surgical efforts with such fixed axes devices cannot beachieved, thereby rendering such instrumentation attempts entirelyunsucessful.

The art contains a variety of attempts at providing instrumentationwhich permit a limited freedom with respect to angulation of the screwand the coupling element. These teachings, however, are generallycomplex, inadequately reliable, and lack long-term durability. Theseconsiderable drawbacks associated with prior art systems also includedifficulty properly positioned the rod and coupling elements, and thetedious manipulation of the many small parts in the operativeenvironment.

It is, therefore, the principal object of the present invention toprovide a pedicle screw and coupling element assembly which provides apolyaxial freedom of implantation angulation with respect to rodreception.

In addition, it is an object of the present invention to provide such anassembly which comprises a reduced number of elements, and whichcorrespondingly provides for expeditious implantation.

Accordingly it is also an object of the present invention to provide anassembly which is reliable, durable, and provides long term fixationsupport.

Other objects of the present invention not explicitly stated will be setforth and will be more clearly understood in conjunction with thedescriptions of the preferred embodiments disclosed hereafter.

SUMMARY OF THE INVENTION

The preceding objects of the invention are achieved by the presentinvention which is a polyaxial locking screw and coupling element foruse with rod stabilization and immobilization systems in the spine. Moreparticularly, the polyaxial screw and coupling element assembly of thepresent invention comprises a bone screw having a head which is curvatein shape, for example semi-spherical, and a two-piece interlockingcoupling element mounted thereto. This combination is mounted inside thebottom of an internal channel of a cylindrical body member.

More specifically, with respect to the cylindrical body member, thetubular body comprises a rod receiving channel formed in the upperportion thereof, with a threading formed on the remaining upper elementsso that a rod securing nut and/or set screw may be threaded thereon oncea rod has been placed in the channel. The body further includes an axialbore which includes extends from the rod receiving channel through tothe bottom of the cylinder. The portion of the axial bore which is belowthe channel forms a receiving chamber, the upper portion thereof havinga constant diameter, and the lower portion of the chamber being inwardlytapered. The inner surface of the upper portion of the chamber and/orthe inner surface of the portion of the axial bore which is above thechamber may further include a threading.

The two-piece interlocking coupling element comprises and socket portionand a cap portion. The socket portion is designed with an interiorsemi-spherical volume, so that it may receive the semi-spherical head ofa corresponding bone screw. The interior volume of the socket portion isopen at both axial ends thereof. The exterior surface of the socketportion, at the bottom thereof, includes a first set of slots whichextend upwardly from the opening so that the interior semi-sphericalvolume may be expanded or contracted by the application of a radialforce. In addition, the exterior surface at the bottom is tapered sothat it is narrower at the bottom than at a midpoint. This taper isdesigned to mate with and nest in the tapered lower portion of thesocket portion of the axial bore of the body member.

The upper exterior surface of the socket portion comprises a second setof slots, directed axially along the element to the midpoint, such thatthe upper opening of the socket element may expand and contract inaccordance with the application of a radial force thereon. The exteriorsurface of this upper section of the socket portion is not tapered andis narrower than the widest taper position of the bottom of the socketportion. The upper section, however, does further include an outwardlyextending annular lip at the uppermost axial position. This uppersection is designed to be inserted into, and joined with, the capportion of the coupling element.

The cap portion has a generally cylindrical shape, having an openbottom. The open bottom is inwardly tapered, forming an inwardlyextending annular lip, so that as the upper end of the socket portion isinserted, its upper slots are narrowed. Once axially inserted beyondthis taper, the upper section of the socket portion expands outward overthe inwardly extending annular lip. The inwardly extending annular lipengages the outwardly extending lip of the socket portion so as toprevent disengagement of the two pieces. The socket portion is thenpermitted to slide into the cap portion, until the larger diameter ofthe tapered lower portion of the socket contacts the entrance of the capportion.

The exterior surface of the cap portion may be threaded, so that it mayengage a threading of the upper portion of the socket portion and/or theinner surface of the axial bore which is above the socket portion. Inaddition, the top of the cap includes an opening so that a screw drivingtool may directly engage the top of the screw.

The assembly of the entire device begins with the joining of the socketportion to the cap portion of the two-piece interlocking couplingelement. This is achieved by the slideable interlocking mating of thetwo elements. Next, the semi-spherical head of the screw is insertedinto the socket portion through the lower expandable opening in thetaper portion. Once these parts have been assembled the screw andcoupling element should be polyaxially rotateable relative to oneanother. The screw and coupling element are then inserted through theaxial bore of the body (which may require the threading the cap portionof the coupling element along the threading on the inner surface of theaxial bore and/or the threading of the cap along the threading of theupper portion of the chamber) until the socket portion nests in thetapered lower portion of the axial bore. If the upper portion of thechamber includes a threading it should not extend beyond the point ofthe initial nesting of the coupling element in the chamber. This isimportant because the cap portion must be able to move relative to thesocket portion.

In this initial position, the top of the cap portion should rest abovethe bottom of the rod receiving channel so that a rod, when placedtherein, seats directly onto the top of the cap. This direct contactprovides the downward force necessary to compress the coupling elementinto the chamber so that the socket portion is compressed in the taperedportion and locks to the head of the screw.

In a preferred variation of this embodiment, the interior surface of thecap portion includes a slight narrowing taper so that as the cap iscompressed downward by the rod, the upper slots of the socket portionare also narrowed, further increasing the crush locking effect on thehead of the screw.

The implantation of this screw by a surgeon may proceed first by theassembly of the screw into its initial state. The shaft of the screw isthen driven into the vertebral bone at the desired angulation. A rod isthen introduced into the rod receiving channel, and the body isangulated into the most ideal position for receiving the rod. A nutand/or set screw is then used to secure the rod in the channel, andsimultaneously to provide a sufficient downward translational force tocause the socket portion to be driven into the tapered portion of thechamber in the axial bore, and further to cause the cap portion to drivedownwardly also (this further compression locking the screw head in theembodiment wherein the sliding of the cap portion toward the socketportion provides an additional compression on the top of the socketportion and therefore onto the head of the screw).

In a preferred variation, the locking nut comprises a cap nut which hasa central post which is designed to provide additional structuralsupport to the inner walls of the element at the top thereof, as well asproviding a central seating pressure point for locking the rod in thechannel. In either variation, the locking nut seats against the rod andprevents it from moving translationally, axially and rotationally.

Multiple screw assemblies are generally necessary to complete the fullarray of anchoring sites for the rod immobilization system, however, thescrew assembly of the present invention is designed to be compatiblewith alternative rod systems so that, where necessary, the presentinvention may be employed to rectify the failures of other systems whenthe surgery may have already begun.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a top view of a human vertebra, which is representative of thetype for which the present invention is useful for coupling thereto arod apparatus;

FIG. 2 is a side view of a pair of adjacent vertebrae of the type shownin FIG. 1;

FIG. 3 is a posterior view of a sequence of vertebrae of the type shownin FIGS. 1 and 2;

FIG. 4 is a side view of a screw having a curvate head which is anaspect of the present invention;

FIG. 5 is a side view of a two-piece interlocking coupling element ofpresent invention;

FIG. 6 is a side view of a two-piece interlocling coupling element ofpresent invention mounted around the head of a screw of the type shownin FIG. 4;

FIG. 7 is a side cross-sectional view of a cylindrical body having achamber for receiving the two-piece interlocking coupling element andthe screw of the present invention;

FIG. 8 is a side cross-sectional view of a top locking nut which is anaspect of the present invention;

FIG. 9 is a side cross-sectional view of an embodiment of the presentinvention in its fully assembled disposition having a rod securelylocked therein; and

FIG. 10 is a side view of an alternative embodiment of the presentinvention in its fully assembled disposition having a rod securelylocked therein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

While the present invention will be described more fully hereinafterwith reference to the accompanying drawings, in which particularembodiments and methods of implantation are shown, it is to beunderstood at the outset that persons skilled in the art may modify theinvention herein described while achieving the functions and results ofthis invention. Accordingly, the descriptions which follow are to beunderstood as illustrative and exemplary of specific structures, aspectsand features within the broad scope of the present invention and not aslimiting of such broad scope.

Referring now to FIG. 4, a side view of the screw portion of the presentinvention, comprising a curvate head, is shown. The screw 120 comprisesa head portion 122, a neck 124, and a shaft 126. In FIG. 4, the shaft126 is shown as having a tapered shape with a high pitch thread 128. Itshall be understood that a variety of shaft designs are interchangeablewith the present design. The specific choice of shaft features, such asthread pitch, shaft diameter to thread diameter ratio, and overall shaftshape, should be made be the physician with respect to the conditions ofthe individual patient's bone, however, this invention is compatiblewith a wide variety of shaft designs.

The head portion 122 of the screw 120 comprises a semi-spherical shape,which has a recess 130 in it. It is understood that the semi-sphericalshape is a section of a sphere, in the embodiment shown the section isgreater in extent than a hemisphere, and it correspondingly exhibits anexternal contour which is equidistant from a center point of the head.In a preferred embodiment, the major cross-section of the semi-sphericalhead 122 (as shown in the two dimensional illustration of FIG. 4)includes at least 270 degrees of a circle.

The recess 130 defines a receiving locus for the application of a torquefor driving the screw 120 into the bone. The specific shape of therecess 122 may be chosen to cooperate with any suitable screw-drivingtool. For example, the recess 130 may comprise a slot for a screwdriver,a hexagonally shaped hole for receiving an alien wrench, or mostpreferably, a threading for a correspondingly threaded post. It isfurther preferable that the recess 130 be co-axial with the generalelongate axis of the screw 120, and most particularly with respect tothe shaft 126. Having the axes of the recess 130 and the shaft 126co-linear facilitates step of inserting the screw 120 into the bone.

The semi-spherical head portion 122 is connected to the shaft 126 at aneck portion 124. While it is preferable that the diameter of the shaft126 be less than the diameter of the semi-spherical head 122, it is alsopreferable that the neck 124 of the screw 120 be narrower than thewidest portion of the shaft 126. This preferable dimension permits thescrew to swing through a variety of angles while still being securelyjoined to the locking collar (as set forth more fully with respect toFIGS. 5, 8-9).

Referring now to FIG. 5, the two elements which form the two-pieceinterlocking coupling element of the present invention are shown in aside cross-section view. Phantom lines show the interior structure ofthe elements along the diametrical cross section. With specificreference to the socket portion 132, the coupling element comprises aroughly cylindrical shape having an interior volume 134 in which thesemi-spherical head 122 of the screw 120 is disposed. The interiorvolume 134 is open at the top 136 of the socket portion 132 and at thebottom thereof 138. The lower section 131 of the socket portion 132comprises a set of slots 133 which extend vertically from the bottom 138of the socket portion 132 to a position above the maximum diameter ofthe semi-spherical interior volume 134. These slots 133 permit theinterior volume to expand and contract in accordance with theapplication of a radial force thereon. The external surface 135 of thelower section 131 of the socket portion 132 is tapered such that thenarrowest part of the lower section 131 is at the bottom 138.

The upper section 139 of the socket portion 132 has a generally constantdiameter, which is less than the diameter at the uppermost position 137of the taper of the lower section 131. A second set of vertical slots141 are provided in this upper section 139 so that it may also expandand contract in accordance with radial forces applied thereto. Inaddition, the uppermost end of this upper section 139 comprises anoutwardly extending annular lip 140.

The cap portion 142 of the coupling element comprises an opening 143 inthe bottom thereof, having an inwardly tapered entrance surfaceconformation 144. As the upper section 139 of the socket portion 132 isinserted into the opening 143 in the cap portion 142, the taper 144 ofthe opening 143 provides an inwardly directed force which causes theupper section 139 to contract (causes the slots 141 to narrow). Thistapered entrance 144 opens to form an annular lip 145 which is usefulfor engaging and retaining the annular lip 140 of the upper section 139of the socket portion 132. The interior surface 146 of the cap portionhas a constant diameter, therein permitting the inserted upper section139 of the socket portion 132 to slide and rotate relative to the capportion 142.

The exterior surface of the cap portion 142 comprises a threading 147which is designed to engage threadings 211 disposed in the axial bore ofthe rod receiving body member (see FIG. 7). In addition, the cap portion142 comprises an axial hole 148 through which a surgeon may insert ascrew driving tool to access the head of the screw which is positionedin the interior volume 134 of the socket portion 132.

More particularly, with respect to the disposition of the head 122 ofthe screw 120 in the socket portion 132, and with reference to FIG. 6, apartially assembled screw 120 and coupling element is shown in a sidecross-section view. The top 136 of the socket portion 132 is insertedinto the opening in the cap portion 142 until the annular lip 140 of thesocket 132 seats into the cap 142. The screw 120 is loosely held withinthe socket 132, which is, in turn, loosely retained within the cap 142.

Referring now to FIG. 7, the rod receiving body member 200 of thepresent invention is shown in a side view, wherein critical features ofthe interior of the element are shown in phantom. The body member 200,which comprises a generally cylindrical tubular body having an axialbore 201 extending therethrough, may be conceptually separated into achamber portion 202 at the bottom of the axial bore 201, and an upperrod receiving channel portion 204, each of which shall be described morefully hereinbelow.

The upper rod receiving channel portion 204 of the body 200 includes achannel 206 formed therein, having rounded bottom surfaces 207. Thechannel 206, in turn, divides the walls of the cylindrical body of theupper portion 204 into a pair of upwardly extending members 214a, 214b.As shown in the embodiment illustrated in FIG. 7, the vertical distancefrom the top 208 of the channel to the curvate bottom 207 thereof, islarger than the diameter of the rod which is to be provided therein.This distance is necessarily larger than the diameter of the rod (seeFIGS. 9 and 10) so that the rod may be fully nested in the channel 206.In addition, the depth of the bottom curvate surface 207 of the channelis such that the cap portion 142 of the two-piece interlocking couplingelement initially seats above the curvate bottom 207 of the body 200.

The upwardly extending members 214a, 214b further have, disposedthereon, a threading 216 (which may be provided on the inner and/orouter circumferential surfaces, but which is shown in FIGS. 7, 9 and 10as being on the inner circumferential surface). This threading 216 isideally suited for receiving a top locking nut (see FIG. 8).

Referring now to the lower portion of the body, the chamber portion 202can further be subdivided into a lower chamber portion 203 whichincludes an inwardly tapered surface, and an upper chamber portion 205which has a constant diameter. The inwardly tapered portion 203 definesa nesting volume into which the socket portion 132 may nest. Prior toits being fully driven into this nesting volume, the socket portion 132and the screw 120 disposed therein may be angulated relative to oneanother, and the screw 120 may be angulated relative to the body 200.Once driven fully into the tapered lower chamber portion 203, however,the taper of the axial bore 201 provides the necessary inwardly directedradial force to cause the socket portion 132 to crush lock to the head122 of the screw 120.

The force which causes the socket portion 132 to be driven downwardlyinto the tapered lower chamber portion 203 is provided by the capportion 142. More specifically, as stated above, when the initiallyassembled screw 120 and coupling element combination 132 and 142 (seeFIG. 6) is advanced into the bottom of the axial bore 201 of the body200, and the socket portion 132 nests in the lower chamber portion 203,the top of the cap portion 142 is positioned to receive the rod (seeFIGS. 9 and 10) directly thereon. The locking of the rod in the channel206 of the body 200 causes the cap portion 142 to be forced downwardlyonto the socket portion 132, which in turn drives the socket portion 132into the tapered lower chamber portion 203 and causes it of compressionlock to the head 122 of the screw 120.

Referring now to FIG. 8, a top locking nut 185 is shown in sidecross-section view. The nut 185 comprises post portion 186 and a flangeportion 187, each of which is rotafionally free, relative to the other.The post portion 186 includes a threading 188 thereon, for engaging andadvancing along a threading 216 on the inner surface of the upwardlyextending members 214a, 214b of the upper portion 204 of the body 200.The bottom surface 189 of the flange portion 187 (which does not rotaterelative to the body as the post portion 186 is rotationally advanced)is intended to seat against the top surface of the rod 250.

Referring now to FIG. 9, in which the fully assembled and body member200, screw 120, coupling element portions 132 and 142, rod 250 andlocking nut 185 are shown in side cross-section views, the implantationof this embodiment is described. First, the screw 120 and the twoportions 132 and 142 of the coupling element are assembled into theirinitial association (see FIG. 6). The combination of the screw 120 andthe two coupling element portions 132 and 142 are then advanced down theaxial bore 201 of the body 200 until the socket portion 132 nests in thelower chamber 203 and the top of the cap portion 142 seats above thebottom 207 of the channel 206. (This insertion of the subassembly of thescrew 120 and coupling element portions 132 and 142 into the axial bore201 of the body 200 may require the threaded advance of the cap portion142 along the interior threads 216 of the body.)

The shaft of the screw 120 is then inserted and driven downward into thevertebral bone at the desired angle. Once properly positioned, the body200 is rotated into the ideal rod receiving position. The rod 250 isthen inserted into the channel 206 and the top locling nut 185 isthreaded onto the threading 216 and compresses the rod 250 to securelylock it in the channel 206. This downward force of the nut 185 and therod 250 onto the cap portion 142 causes the cap portion to translatedownward thus causing the socket portion 132 to translate downward inthe tapered chamber 203 and contract to crush against the head 122 ofthe screw 120. The assembly is thereby fully locked in position.

Referring to FIG. 10, a variation of the above device is shown in asimilar cross-section view. In this embodiment, the inner surface 146′of the cap portion 142 is tapered inwardly in the vertical direction sothat the downward translation of the cap portion 142 causes the annularlip 140 of the socket portion 132 to be compressed inwardly. This causesthe slots 141 of the upper section 139 of the socket portion 132 tonarrow. This may be utilized to further clamp the interior volume 134against the head 122 of the screw 120.

While there has been described and illustrated embodiments of apolyaxial screw and coupling element assembly for use with posteriorspinal rod implantation apparatus, it will be apparent to those skilledin the art that variations and modifications are possible withoutdeviating from the broad spirit and principle of the present invention.The present invention shall, therefore, be limited solely by the scopeof the claims appended hereto.

1. A polyaxial screw and coupling element assembly for use withorthopedic rod implantation apparatus, comprising: a screw having asemi-spherical head; a cylindrical body including an axial bore defininga bottom chamber portion at a bottom end thereof and a rod receivingchannel at a top end thereof, said bottom chamber portion furtherdefining a tapered lower portion and a constant diameter upper chamberportion, and said top end having a threading thereon; a two-pieceinterlocking coupling element including a socket portion having asemi-spherical interior volume for receiving therein the head of saidscrew, upper and lower sections, and vertical slots formed in said upperand lower sections, at least one of said slots rendering said interiorvolume expandable and contractable, said lower section having a taperedexterior surface for nesting in said tapered lower chamber portion ofsaid cylindrical body such that forceable advancement of socket portionalong the tapered lower chamber portion of the axial bore causes the atleast one of said slots to narrow such that the semi-spherical interiorvolume contract, and a cap portion having an opening in a bottom thereofand an interior chamber extending upwardly therefrom for joining with,and slideably retaining therein, the upper section of said socketportion, said cap portion initially seating with a portion thereofextending into a bottom portion of said rod receiving channel; and a toplocking nut, mateable with said threading, for locking a rod in saidchannel and for applying therethrough a downward force onto said capportion, wherein said semi-spherical head portion is rotationally freelymounted within the semi-spherical interior volume of the socket portionprior to said socket portion being forceably advanced into the taperedlower chamber portion of the axial bore, and whereby downwardcompression of a rod in said channel portion of said body member, bysaid top locking nut, onto the cap portion causes the forceableadvancement of the socket portion into the tapered lower chamber portionof the axial bore, and locks the screw, coupling element and bodyrelative to one another.
 2. The polyaxial screw as set forth in claim 1,wherein said semi-spherical head of said screw further includes a recessformed therein for receiving therein a screwdriving tool such that saidscrew may be threadably advanced into a vertebral bone.
 3. The polyaxialscrew as set forth in claim 1, wherein said threading on said top end ison the interior surface of said channel.
 4. The polyaxial screw as setforth in claim 3, wherein said cap portion further includes a threadingand wherein said cap portion needs to be threadably advanced along thethreading to be seated in the bore into its initial position.
 5. Theassembly as set forth in claim 1, wherein said socket portion furthercomprises a substantially constant diameter upper section having anoutwardly annular extending lip at an extreme end thereof, wherein saidopening in the bottom of the cap portion comprises an inwardly directedannular lip, and wherein at least one of said vertical slots in theupper section of said socket portion renders the upper section thereofto be expandable and contractable such that the upper section of thesocket portion may be forceably inserted into the opening in the bottomof the cap portion so that it may be retained in the interior chambertherein by mutual interference engagement of the inwardly directedannular lip of the cap portion and the outwardly extending annular lipof the socket portion.
 6. The assembly as set forth in claim 5, whereinthe interior chamber of the cap portion comprises a tapered surface suchthat advancement thereof into the hole causes an inwardly directed forceagainst the upper section of the socket portion, therein causing the atleast one of said vertical slots in the upper section to narrow andcauses the upper section to contract and further lock the head of thescrew within the interior semi-spherical volume of the socket portion.7. An orthopaedic implant apparatus having a rod and a plurality ofscrew and coupling element assemblies, comprising: at least one screwhaving a semi-spherical head; at least one cylindrical body including anaxial bore defining a bottom chamber portion at a bottom end thereof anda rod receiving channel at a top end thereof, said bottom chamberportion further defining a tapered lower portion and a constant diameterupper chamber portion, and said top end having a threading thereon; atleast one corresponding two-piece interlocking coupling elementincluding a socket portion having a semi-spherical interior volume forreceiving therein the head of said corresponding screw, upper and lowersections, and vertical slots formed in said upper and lower sections, atleast one of said slots rendering said interior volume expandable andcontractable, said lower section having a tapered exterior surface fornesting in said corresponding tapered lower chamber portion of saidcylindrical body such that forceable advancement of socket portion alongthe tapered lower chamber portion of the axial bore causes the at leastone of said slots to narrow such that the semi-spherical interior volumecontract, and a cap portion having an opening in a bottom thereof and aninterior chamber extending upwardly therefrom for joining with, andslideably retaining therein, the upper section of said socket portion,said cap portion initially seating with a portion thereof extending intoa bottom portion of said rod receiving channel; and at least onecorresponding top locking nut, mateable with said threading, for lockinga rod in said channel and for applying therethrough a downward forceonto said cap portion, wherein said semi-spherical head portion isrotationally freely mounted within the semi-spherical interior volume ofthe socket portion prior to said socket portion being forceably advancedinto the tapered lower chamber portion of the axial bore, and wherebydownward compression of a rod in said channel portion of said bodymember, by said top locking nut, onto the cap portion causes theforceable advancement of the socket portion into the tapered lowerchamber portion of the axial bore, and locks the screw, coupling elementand body relative to one another.
 8. The apparatus as set forth in claim7, wherein said threading on said top end is on the interior surface ofsaid channel.
 9. The apparatus as set forth in claim 8, wherein said capportion further includes a threading and wherein said cap portion needsto be threadably advanced along the threading to be seated in the boreinto its initial position.
 10. The apparatus as set forth in claim 7,wherein said socket portion further comprises a substantially constantdiameter upper section having an outwardly annular extending lip at anextreme end thereof, wherein said opening in the bottom of the capportion comprises an inwardly directed annular lip, and wherein at leastone of said vertical slots in the upper section of said socket portionrenders the upper section thereof to be expandable and contractable suchthat the upper section of the socket portion may be forceably insertedinto the opening in the bottom of the cap portion so that it may beretained in the interior chamber therein by mutual interferenceengagement of the inwardly directed annular lip of the cap portion andthe outwardly extending annular lip of the socket portion.
 11. Theapparatus as set forth in claim 10, wherein the interior chamber of thecap portion comprises atapered surface such that advancement thereofinto the hole causes an inwardly directed force against the uppersection of the socket portion, therein causing the at least one of saidvertical slots in the upper section to narrow and causes the uppersection to contract and further lock the head of the screw within theinterior semi-spherical volume of the socket portion.
 12. An orthopedicdevice for securing immobilizing structures to sequences of bones,comprising: a screw having a semi-spherical head and a threaded shaft; acoupling element having an axial hole extending therethrough, a portionof the axial hole defining an interior volume for receiving therein thesemi-spherical head of the screw such that the threaded shaft may bemoved through a variety of angles relative to the axial hole, thecoupling element further including at least one slot rendering at leastthe interior volume deformable, the coupling element further including atapered exterior surface; a receiving member including a through holehaving an interior wall surface, a portion of the interior wall surfaceof the through hole being shaped to receive the coupling element and thescrew when the semi-spherical head of the screw is mounted within thecoupling element; and wherein advancement of the screw through thethrough hole relative to the receiving member when the exterior surfaceof the coupling element engages the interior wall surface of the throughhole prevents the coupling element from further advancement through thehole, preventing the semi-spherical head of the screw from advancingfurther through the through hole, and causing locking of the screwrelative to both the coupling element and to the receiving memberthereby locking the angle of the screw relative to the axial hole. 13.The assembly as set forth in claim 12, wherein the semi-spherical headof the screw further includes a recess formed therein for receivingtherein a screwdriving tool such that the screw may be advanced into avertebral bone.
 14. An orthopedic implant apparatus comprising: afixation element having a semi-spherical head and a shaft extendingtherefrom; a receiving member including an axial bore defined by aninterior surface wall, a portion of the axial bore having a taperedportion; a socket portion having a semi-spherical interior volume forreceiving therein the semi-spherical head, and an exterior surfacecapable of nesting against the interior surface wall of the taperedportion, the socket portion being located in the axial bore of thereceiving member; wherein the semi-spherical head is rotationally freelymounted within the semi-spherical interior volume of the socket portionprior to the socket portion being forcibly advanced against the interiorsurface wall of the tapered portion, and whereby advancement of thesocket portion causes the fixation element, the socket portion and thereceiving member to be locked relative to one another.
 15. The assemblyas set forth in claim 14, wherein the semi-spherical head of thefixation element further includes a recess formed therein for receivingtherein a screwdriving tool such that the fixation element may beadvanced into a vertebral bone.
 16. A pedicle screw and rod couplingmember assembly wherein said rod coupling member and screw are capableof being selectively positioned and locked at a plurality of anglesrelative to one another, said assembly comprising: a bone screw having acurvate head; a locking collar disposed around said curvate head andhaving a slot; and a rod coupling member having a bore therethrough forreceiving said curvate head and locking collar, said bore having aninterior surface and a pair of upwardly extending members forming arod-receiving U-shaped channel, said upwardly extending members havingthreads thereon for receiving a locking device, wherein said pediclescrew and rod coupling member assembly assume a first position such thatsaid rod coupling member and screw are capable of assuming a variety ofangles relative to each other and a second position with a rod memberdisposed in said rod-receiving U-shaped channel and said locking devicetightened on said threads such that said locking collar engages said rodcoupling member interior surface to lock the angle of the screw relativeto the rod coupling member.
 17. The orthopedic device as set forth inclaim 16, wherein the curvate head of the screw further includes arecess formed therein for receiving therein a screwdriving tool suchthat the screw may be advanced into a vertebral bone.
 18. The orthopedicdevice as set forth in claim 16, wherein the rod coupling memberinterior surface includes a lower portion that is inwardly tapered, andwhich engages said locking collar to crush lock the locking collar tothe curvate head of the screw.
 19. The orthopedic device as set forth inclaim 16, wherein the rod coupling member interior surface includes alower portion that is inwardly tapered, and the exterior surface of thelocking collar is tapered to engage said inwardly tapered surface. 20.An orthopedic implant and bone anchoring member assembly wherein saidbone anchoring member is selectively positionable and lockable at anyone of a plurality of angles relative to said implant, said assemblycomprising: a bone anchoring member having a curvate head and a threadedshaft; a rod coupling member having a bore extending therethrough forreceiving said curvate head in a lower portion thereof, said bore havingan interior surface; a rod positioned in an upper portion of said bore;and a locking ring positioned in the lower portion of the bore betweensaid curvate head and said interior surface, wherein axial translationof said rod relative to said bore urges said curvate head into lockingengagement with said locking ring to lock the angle of the screwrelative to rod coupling element.
 21. The assembly of claim 20, whereinsaid curvate head is semi-spherical.
 22. The assembly of claim 20,wherein said bore includes at least two portions having differentdiameters.
 23. The assembly of claim 22, wherein said locking ring ispositioned at least partially within one of said portions havingdifferent diameters.
 24. The assembly as set forth in claim 20, whereinsaid locking ring circumferentially retains the curvate head.
 25. Theassembly as set forth in claim 20, further comprising a locking devicefor engaging said rod to urge said rod to translate axially relative tosaid bore.
 26. The assembly as set forth in claim 25, wherein saidlocking device comprises a nut engageable with threads on saidorthopedic implant.
 27. The assembly of claim 20, wherein the boreincludes a tapered portion and the locking ring contacts against theinterior surface of the tapered portion.
 28. An orthopedic device forsecuring structures to bone, comprising: a screw having a curvate headand a threaded shaft; a coupling element having an axial hole extendingtherethrough, a portion of the axial hole defining an interior volumefor receiving therein the curvate head of the screw such that thethreaded shaft may be moved through a variety of angles to the axialhole, the coupling element further including at least one slot renderingat least the interior volume deformable, the coupling element furtherincluding a tapered exterior surface; a receiving member including athrough hole having an interior wall surface, a portion of the interiorwall surface of the through hole being shaped to receive the couplingelement and the screw when the curvate head of the screw is mountedwithin the coupling element; and wherein advancement of the screwthrough the through hole relative to the receiving member when theexterior surface of the coupling element engages the interior wallsurface of the through hole prevents the coupling element from furtheradvancement through the hole, and causing locking of the screw relativeto both the coupling element and to the receiving member thereby lockingthe angle of the screw relative to the axial hole.
 29. The assembly asset forth in claim 28, wherein the curvate head of the screw furtherincludes a recess formed therein for receiving therein a screwdrivingtool such that the screw may be advanced into a vertebral bone.
 30. Anorthopedic implant apparatus comprising: a fixation element having acurvate head and a shaft extending therefrom; a receiving memberincluding an axial bore defined by an interior surface wall, a portionof the axial bore having a tapered portion; a socket portion having acurved interior volume for receiving therein the curvate head, and anexterior surface capable of nesting against the interior surface wall ofthe tapered portion, the socket portion being located in the axial boreof the receiving member, the socket portion including a slot with theinterior volume being deformable when the socket portion moves along thetapered portion; wherein the curvate head is rotationally freely mountedwithin the curved interior volume of the socket portion prior to thesocket portion being forcibly advanced against the interior surface wallof the tapered portion, and whereby advancement of the socket portioncauses the fixation element, the socket portion and the receiving memberto be locked relative to one another.
 31. The assembly as set forth inclaim 30, wherein the curvate head of the fixation element furtherincludes a recess formed therein for receiving therein a screwdrivingtool such that the fixation element may be advanced into a vertebralbone.